1. Field of the Invention
This invention relates to a wavelet inverse transform method and apparatus used in extracting and decoding only transform coefficients of a specified partial area from wavelet transform coefficients obtained on encoding by wavelet transform. This invention also relates to a corresponding wavelet decoding method and apparatus.
2. Description of Related Art
Among conventional typical picture compression systems, there is a JPEG (Joint Photographic Experts Group) system, standardized by the ISO (International Organization for Standardization). This system, employing an orthogonal transform, specifically the discrete cosine transform (DCT), is known to furnish a good encoded or decoded picture when a higher number of allocated bits is used. However, if the number of encoding bits is decreased to more than a certain extent, block distortion proper to DCT becomes outstanding so that subjective deterioration is apparent. On the other hand, such a system in which an entire picture is split into plural bands by a filter, termed a filter bank, comprising the combination of a high-pass filter and a low-pass filter, and encoding is performed from one band to another, is now being researched briskly. In particular, the wavelet encoding free from the defect of block distortion, which becomes outstanding in high compression such as is presented in DCT, is thought to be promising as a technique which should take the place of the DCT.
Nowadays, an electronic still camera or a video movie exploits the JPEG or MPEG, with the transform system being the DCT. Since a product based on the wavelet transform is predicted to be presented to the market in time to come, investigations in improving the efficiency in the encoding system are proceeding briskly in many research institutes. As a matter of fact, JPEG 2000, now being worked by ISO/IEC/JTC1_SC29/WG1, which is the same organization as the JPEG, as a format the recommendation for the standardization of which is scheduled to be issued in December 2000, is felt to be promising as the next-generation international standardization system for still pictures. With this JPEG 2000, it has almost been established to use the wavelet transform in place of the preexisting DCT of JPEG as a basic transformation system for picture compression.
The present invention is directed to the elimination of the problem in expanding only a partial area in wavelet inverse transform. That is, the entire wavelet transform coefficients are not read out and decoded, as is done in the conventional technique. This represents a significant merit in reducing the memory capacity size. In the wavelet encoding, wavelet transform needs to be applied to the entire picture to store and hold the generated wavelet transform coefficients transiently in a memory. In wavelet decoding, the operation which is the reverse of the wavelet encoding operation is performed, thus necessitating an extremely large memory capacity for storing and holding the coefficients for the entire picture. Should the picture size be increased, the memory capacity needs to be correspondingly increased. Thus, the conventional practice is not desirable for a device having a limited memory capacity, such as an electronic still camera, camcorder or PDA.
Recently, in e.g., the international standardization activities of JPEG 2000, such a technique is investigated in which the entire picture of an object of encoding is split into plural blocks to perform the encoding on the block basis. If the encoding by the encoder is done on the block bases from the outset, partial decoding can be achieved by reading out an encoded bitstream associated with a pre-set block. However, there lacks up to now a research into partial decoding in the absence of constraint on the encoder.